The brittlestar, a relative of the starfish, seems to be able to flee from predators in the murky ocean depths without the aid of eyes. Now scientists have discovered its secret: its entire skeleton forms a big eye.
A new study shows that a brittle star species called Ophiocoma wendtii has a skeleton with crystals that function as a visual system, apparently furnishing the information that lets the animal see its surroundings and escape harm. The brittlestar architecture is giving ideas to scientists who want to build tiny lenses for things like optical computing.
"This study shows how great materials can be formed by nature, far beyond current technology," said Dr. Joanna Aizenberg, a material scientist at Lucent Technologies' Bell Laboratories and the lead author of the study.
"They form very interesting unique structures that have interesting mechanical and optical applications," Dr. Aizenberg said. "They form nearly perfect microlenses."
The study, published on Aug. 23 in the journal Nature, was conducted by an international team that included material scientists, theoretical physicists, chemists and biologists.
The brittlestar, also known as the serpent star, is an invertebrate that has five waving arms attached to a disk.
Researchers were puzzled because the brittlestars not only appeared to be able to see without eyes, but also seemed to see better than some other primitive marine organisms.
The brittlestars move quickly to catch prey and change color from dark brown in daytime to gray at night.
"This type of brittlestar seems to be much more sensitive to light," Dr. Aizenberg said, "as they detect predators and locate hideouts."
Dr. Sonke Johnsen of Woods Hole National Oceanographic Institution, who was not involved in the study, said: "It was suspected that brittle- stars were one big compound eye. This looks like it's the case."
The brittlestars secrete a crystalline form of calcium carbonate called calcite and organize it to make crystals in any shape or form.
"It is an observation of a phenomenon never seen before, the use of a crystal lattice to focus light," said Dr. Steve Weiner of the structural biology department of the Weizmann Institute of Science in Israel and an author of the study.
The only other crystals like these in nature appeared to be in the eyes of the long-extinct marine organisms called trilobites, Dr. Weiner said.
The researchers investigated the skeletal structures of the light-sensitive species of brittlestar and of others that do not respond to light. They found that only the light-sensitive species had the layer of lenses.
The study used optical lithography, a technique used for computer chip etching that is similar to the childhood experiment of taking a lens on a sunny day to concentrate light and burn holes through paper. Brittlestar crystals were placed above photosensitive material, and light was shined through them.
The crystals acted as lenses, collecting light and focusing it on points that corresponded to nerve bundles, part of the brittlestar's diffuse nervous system.
The brittlestar lenses optimize light coming from one direction, and the many arrays of them seem to form a compound eye, said Dr. Alexei Tkachenko of Bell Laboratories, another author of the study.
The beadlike lenses of these brit tlestars focus light at least 10 times as well as the microlenses now made in laboratories. Since the brittlestar lenses are single crystals, there is no distortion.
Optical computing could eventually benefit tremendously from such microlenses, said Dr. Daniel Morse, chairman of the biomolecular science and engineering program at the University of California at Santa Barbara.
Arrays of microlenses that imitated the brittlestar's, Dr. Morse said, may someday be used to build systems that will "become more competitive with other electronic or magnetic systems that are the basis for the computing and telecommunication systems in use today."
Dr. Aizenberg pointed out that the brittlestar appeared to have solved a problem that had stumped technology. "In general," she said, "arrays of microlenses are something that technology tried a couple of years ago. Nobody knew something like that already existed in nature."
File Date: 09.12.01